A data-storage system often includes a volatile cache for temporary storage of data that will ultimately be written to a disk. When a host requests that particular data be written, the data-storage system writes that data to the cache and notifies the host that the write is complete. A short time later, and without further interaction with the host, the data-storage system copies the data from the cache to a disk. Because a write to disk is so much slower than a write to cache, this two-stage procedure for data-storage has the advantage of concealing from the host the latency associated with writing to a disk.
A disadvantage of this two-stage procedure is that for a brief interval, the data only exists in a volatile memory. During this interval, an unexpected power interruption may result in loss of that data.